1. Technical Field of the Invention
The present invention relates to a wear resistant member mainly composed of silicon nitride and a method of manufacturing the member, and more particularly to a silicon nitride wear resistant member and a method of manufacturing the member capable of exhibiting excellent wear resistance, particularly rolling life characteristics when the wear resistant member is used as rolling bearing member, and is suitable as a material for constituting a rolling bearing member requiring an excellent durability.
2. Description of the Related Art
Wear resistant member (abrasion resistant member) has been applied to various fields such as, for example, bearing member, various roller members for rolling operation, compressor vane, gas-turbine blade, engine parts such as cam roller or the like. As a material for constituting the wear resistant member, various ceramic materials have been conventionally used. In particular, silicon nitride sintered body is excellent in mechanical strength and wear resistance property. For this reason, the silicon nitride sintered body has been applied to various technical fields.
Various sintering compositions for the silicon nitride sintered bodies are well known: such as silicon nitride/yttrium oxide/aluminum oxide system; silicon nitride/yttrium oxide/aluminum oxide/aluminum nitride system; and silicon nitride/yttrium oxide/aluminum oxide/titanium oxide system or the like. Sintering assistant agents composed of the oxides of rare earth elements, such as yttrium oxide (Y2O3) in the sintering compositions listed above, have a function of generating grain boundary phase (liquid phase) composed of Si-rare earth element-Alxe2x80x94Oxe2x80x94N or the like during the sintering operation. Therefore, the sintering assistant agents are added to a material composition for enhancing the sintering characteristics of sintering materials, and achieve high density and high strength of the sintered bodies.
According to the conventional art, silicon nitride sintered bodies are generally mass-produced as follows. After a sintering assistant agent as mentioned above is added to the material powder of silicon nitride, the material mixture is molded to form a compact. Thus obtained compact is then sintered in a sintering furnace at about 1,650-1,900xc2x0 C. for a predetermined period of time followed by naturally cooling (self-cooling) the resultant sintered body in the furnace at a high cooling rate.
Among the various applications to the wear resistant members using the above silicon nitride sintered body, the bearing member has been generally recognized to be useful material. Such bearing member has been used to various applications, and also started to be reviewed to use as an important protection safety parts. For this reason, the bearing member composed of silicon nitride sintered body i.e. rolling bodies such as ball and roller or the like has been required to further improve reliability.
For example, defects such as flaw and crack or the like formed on a surface of the rolling body will result to a breakage of not only the bearing member per se but also an entire system using the bearing member. Therefore, there is adopted a manufacturing process for excluding or eliminating such defects as completely as possible. In a similar way, a pore existing close to the surface of the rolling body would also be a cause of deteriorating the reliability of the bearing member, so that the pore is removed at a process when the member is worked into a product having a final shape.
Although the silicon nitride sintered body produced by the conventional method achieves an improved bending strength, fracture toughness and wear resistance, however, the improvement is insufficient. A durability as a rolling bearing member requiring a particularly excellent sliding property is insufficient, so that a further improvement has been demanded.
In these days, a demand of ceramic material as precision device members has increased. In these applications, advantages such as high hardness and light weight together with high corrosion resistance and low thermal expansion property of the ceramic are utilized. In particular, in view of the high-hardness and high wear resistance, application as a wear resistant member for constituting a sliding portion of the bearing or the like has been rapidly extended.
However, in a case where rolling balls of a bearing or the like were constituted by the wear resistant member composed of ceramic, when the rolling balls were rolled while being repeatedly contacted with counterpart at a high stress level, the rolling life of the wear resistant member was not sufficient yet. Therefore, a surface of the wear resistant member is peeled off and the member causes cracks, so that the defective member was liable to causes vibration and damage to a device equipped with the bearing. At any rate, there had been posed a problem that the durability and reliability as a material for constituting the parts of the device was low.
The silicon nitride sintered body produced by the conventional manufacturing method was inevitably formed with defects such as flaws and cracks at a stage after sintering operation, the defects were formed at portions not only the surface of the sintered body but also a deep portion so that the defects extend to a relatively inner portion of the sintered body. These defects lead to defective products, or even if the defects do not lead to the defective products, a manufacturing manpower for obtaining a surface having a high reliability by removing the cracks or the like i.e. a manpower required for a process of a surface grinding operation till the cracks or the like were substantially eliminated was disadvantageously increased, thus leads to an increase of the manufacturing cost of the rolling bodies.
That is, when a silicon nitride molded body is sintered, a part of impurity oxygen contained in the silicon nitride powder (material powder) and a part of oxygen contained in the sintering assistant agent are evaporated thereby to generate a gas component. The gas component is simultaneously generated at an almost the same time when the silicon nitride molded body starts to shrink at the sintering operation. In ordinary sintering method, since a densification starts at a surface portion of the sintered body in accordance with the start of the shrinkage of the molded body, it is difficult to remove the gas component contained in the inner portion of the sintered body.
When the above gas component remains in the sintered body, pores are formed in the silicon nitride sintered body, and oxygen is combined with Si thereby to remain as SiO2. In the conventional manufacturing method, since the gas component could not be sufficiently removed, the pores and SiO2 resulting from the gas component remain in a relatively broad region, and these pores and SiO2 caused a crack or the like extending towards inner portion of the sintered body. In order to remove the cracks, it was required to remove the surface of the sintered body to some depth extent, so that these removal operations invites the occurrence of defects and an increase of the manufacturing cost of the sintered body.
Further, for example, when the sintered body is subjected to HIP treatment thereby to fill the pores with a liquid phase formed by the sintering assistant agent, the resultant silicon nitride sintered body can be highly densified. However, a segregation of the liquid phase component are disadvantageously formed to a portion where the pores were existing. The liquid phase components have strength and hardness lower than those of silicon nitride crystal grains, so that the segregation of the liquid phase component becomes a starting point of a breakage when the silicon nitride sintered body is used as the wear resistant member. Accordingly, such an aggregated substance is also required to be removed.
At any rate, in the conventional method of manufacturing the silicon nitride sintered body, there has been posed a problem that pores, cracks, segregations of liquid phase resulting from the gas component are unavoidably distributed to some depth extent of the sintered body. These defects would lead to defective product and also lead to an increase of working margin (a depth to be removed by working) of the surface of sintered body for the purpose of obtaining a good surface capable of increasing the reliability of the rolling body. These factors would increase the manufacturing cost of the wear resistant members such as rolling body or the like.
The present invention had been achieved for solving the aforementioned problems. Accordingly, an object of the present invention is to provide a wear resistant member and a method of manufacturing the member particularly excellent in sliding characteristics in addition to high strength property and high toughness property.
In particular, the object of the present invention is to provide a wear resistant member and a method of manufacturing the member capable of easily removing the gas component to be a cause of increasing defects and manufacturing cost, and capable of reducing the manufacturing cost in addition to realize various characteristics required for the wear resistant member.
In order to attain the objects described above, the inventors of the present invention had studied the effects and influences of the types of silicon nitride powder, sintering assistant agent and additives, the amounts thereof, and the sintering conditions on the characteristics of the final products, that is, the sintered bodies, by performing experiments.
As a result, the experiments provided the following findings. That is, a wear resistant member composed of a silicon nitride sintered body having a particularly excellent rolling life of sliding property could be obtained in addition to high strength property and a high toughness property: when certain amounts of a rare earth element, alumina, if necessary, magnesia, aluminum nitride, titan oxide or the like were added to a fine material powder of silicon nitride to prepare a material mixture; followed by molding the material mixture to form a molded compact (molded body) and degreasing the compact, and the compact was subjected to a deoxidization treatment (reduction of oxygen content) by holding the compact under a predetermined conditions on the way to sintering step, followed by subjecting a main sintering operation; or when the sintered body after completion of the sintering operation was subjected to a hot isostatic pressing (HIP) treatment under predetermined conditions.
Further, when the sintered body after completion of the sintering operation was moderately cooled at a cooling rate of 100xc2x0 C. per hour or lower within a temperature range from the above high sintering temperature to a temperature at which the liquid phase generated from rare earth element during the sintering operation solidifies, it was also confirmed that a pore size to be formed in the sintered body structure could be further minimized.
In addition, prior to attaining a temperature (1600 to 1850xc2x0 C.) at which a final densification-sintering of the silicon nitride was started, when the molded compact was heated to a temperature to some extent and held the compact at such temperature for a predetermined period of time, so that gas components such as oxygen and SiO2 contained in the sintered body could be migrated towards outside and discharged from the sintered body. In view of this fact, it was found that the oxygen content of an outer peripheral portion of the sintered body could be finally lowered. Namely, a content of gas component, which became a cause of allowing the pores and cracks to further invade into an inner portion of the sintered body, could be reduced.
The present invention had been achieved on the basis of the aforementioned findings.
That is, according to the present invention, there is provided a wear resistant member composed of silicon nitride sintered body containing 1 to 10 mass % of rare earth element in terms of oxide thereof as sintering agent, wherein a total oxygen content of said silicon nitride sintered body is 6 mass % or less, a porosity of said silicon nitride sintered body is 0.5 vol. % or less, and a maximum size of pore existing in grain boundary phase of the silicon nitride sintered body is 0.3 xcexcm or less.
Further, in the above wear resistant member, it is preferable that the silicon nitride sintered body has a three point bending strength of 900 MPa or more and a fracture toughness of 6.5 MPaxc2x7m1/2 or more, and a rolling life defined as a rotation number of steel balls rolling along a circular track formed on the wear resistant member formed of the silicon nitride sintered body until a surface of the silicon nitride wear resistant member peels off is 1xc3x97108 or more, when the rolling life is measured in such a manner that a circular track having a diameter of 40 mm is set on the wear resistant member, three rolling balls each having a diameter of 9.525 mm and composed of SUJ2 are provided on the circular track, and the rolling balls are rotated on the track at a rotation speed of 1200 rpm under a condition of being applied with a pressing load of 400 Kg.
Furthermore, in the above wear resistant member, it is preferable that the silicon nitride sintered body has a crash strength of 200 MPa or more and a fracture toughness of 6.5 MPaxc2x7m1/2 or more, and a rolling fatigue life defined as a time until a surface of rolling balls composed of the silicon nitride wear resistant member rolling along a circular track formed on a steel plate peels off is 400 hours or more, when the rolling fatigue life is measured in such a manner that three rolling balls each having a diameter of 9.525 mm are formed from the silicon nitride wear resistant member, the three rolling balls are provided on the circular track having a diameter of 40 mm set on the steel plate formed of SUJ2, and the rolling balls are rotated at a rotation speed of 1200 rpm on the track under a condition of being applied with a pressing load so as to impart a maximum contact stress of 5.9 GPa to the balls.
In this regard, as a method of measuring the wear resistance (rolling fatigue life) in a case where the wear resistant member has a ball-shape, the present invention is specified by using a ball composed of the wear resistant member having a diameter of 9.525 mm (xe2x85x9c inch) as a standard basis. However, the wear resistant member of this invention is not limited to the ball having the above size.
For example, in a case where the size of ball is different from the diameter of 9.525 mm (xe2x85x9c inch), the wear resistance is measured after the maximum contact stress is changed in accordance with the size of the ball. In this case of changing the maximum contact stress, the maximum contact stress for the ball having a different-size can be calculated in proportional to the size of the ball by using a unit conversion formula of 1 Pa=1.02xc3x9710xe2x88x925 kgf/cm2. Note, even if the size of the ball is different from the above standard size, the rolling fatigue life of 400 hours or more can be obtained as far as the balls are composed of the wear resistant member of the present invention.
Further, in the wear resistant member according to the present invention, it is preferable that the silicon nitride sintered body contains at most 5 mass % of at least one of aluminum and magnesium in terms of the amount of oxide thereof. Furthermore, it is also preferable that the silicon nitride sintered body contains at most 5 mass % of aluminum nitride. In addition, it is also preferable that the silicon nitride sintered body contains at most 5 mass % of at least one element selected from the group consisting of Ti, Hf, Zr, W, Mo, Ta, Nb and Cr in terms of oxide thereof.
Further, when the wear resistant member composed of above the silicon nitride sintered body is a rolling bearing member, the wear resistant member can particularly exhibit excellent sliding property and durability.
Further, in above the wear resistant member according to the present invention, it is preferable that the silicon nitride sintered body is mainly composed of silicon nitride and contains a little amount of oxygen, the silicon nitride sintered body comprises an outer peripheral portion having a lower oxygen content than that of a center portion, and a difference of the oxygen contents between the outer peripheral portion and the center portion is within a range of 0.2-2 mass %. Furthermore, it is also preferable that the silicon nitride sintered body has a Vickers"" hardness of 1200 or more.
When the oxygen content (oxygen concentration) at outer peripheral portion of the silicon nitride sintered body is reduced to be low as described above, an inversion depth (penetration depth) of the defects such as pores and/or cracks can be decreased to be shallow. Therefore, the generation of defective product resulting from the defects can be effectively suppressed, and the cost and manpower required for the surface working the sintered body can be also reduced. In view of these facts, when the wear resistant members such as rolling body or the like are prepared from the silicon nitride sintered body, the manufacturing cost can be remarkably reduced.
Further, it is also preferable that a difference in content of metal contained as sintering agent in the outer peripheral portion and the center portion of the silicon nitride sintered body used in the present invention is 0.2 mass % or less. As described above, in the silicon nitride sintered body used in the present invention, a removal of unnecessary oxygen is realized while a distribution of other metal components is substantially the same as those of conventional sintered bodies, so that inherent characteristics (strength and sliding property) as silicon nitride sintered body are maintained to be unchanged.
In the silicon nitride sintered body used in the present invention, when the difference in the oxygen contents of the outer peripheral portion and the center portion is within the range of 0.2 to 2 mass %, above functions and effects can be obtained. However, when the oxygen content of an entire sintered body is excessively high, there may be a fear that inherent characteristics of the silicon nitride sintered body are deteriorated. Therefore, it is preferable to set the oxygen content of the entire sintered body to 6 mass % or less. At most 4.5 mass % is particularly preferable.
Further, in above the wear resistant member, it is preferable that the silicon nitride sintered body comprises an intermediate portion of which oxygen content is at most 1 mass % lower than that of the center portion. When such intermediate portion is provided to the sintered body, the gradient of the oxygen content can be further moderated, whereby a defect ratio of the member and the cost required for surface working can be further suppressed.
A method of manufacturing the wear resistant member according to the present invention preferably comprises the steps of: molding a material composition mainly composed of silicon nitride powder to form a molded compact; heating the compact obtained in above molding step to a temperature of 1,200-1,500xc2x0 C. in a vacuum atmosphere of 0.01 Pa or less and holding the compact at the temperature of 1,200-1,500xc2x0 C. for 1-10 hours thereby to conduct a vacuum treatment; and sintering the compact subjected to the vacuum treatment at a temperature range of 1,600-1,800xc2x0 C. in nitrogen gas atmosphere thereby to form a wear resistant member composed of silicon nitride sintered body.
The wear resistant member according to the present invention is characterized by being composed of the silicon nitride sintered body as prepared above, and is effective to bearing members such as bearing ball or the like. In particularly, the wear resistant member is effective to a large bearing ball having a diameter of 9 mm or more.
As a preferable embodiment of the present invention, the silicon nitride sintered body is mainly composed of silicon nitride and contains a little amount of oxygen, and the silicon nitride sintered body comprises an outer peripheral portion having an oxygen content lower than that of a center portion, and a difference of the oxygen contents between the outer peripheral portion and the center portion is within a range of 0.2-2 mass %. That is, the outer peripheral portion is constituted as a low-oxygen-content region of which oxygen content is reduced to be 0.2-2 mass % lower than that of the center portion of the silicon nitride sintered body.
FIG. 2 is a view schematically showing respective regions each having a different oxygen content in the silicon nitride sintered body used in the present invention. Note, FIG. 2 exemplarily shows a silicon nitride sintered body 2 having a ball-shape as one example, and the present invention is not limited thereto. In the present invention, a region ranging from a center O of the silicon nitride sintered body 2 to a potion apart from the center O at a distance of 5% of radius R is defined as a center portion A. While, a region ranging from an outer surface S of the silicon nitride sintered body 2 to a potion apart from the outer surface S at a distance of 1% of radius R is defined as an outer peripheral portion B. In other words, the outer peripheral portion B is defined as a region ranging from a point apart from center O at a distance of 99% of the radius R to a point apart from center O at a distance of 100% of the radius R. In this connection, when the wear resistant member has a plate-shape, the respective regions are defined on the basis of a thickness of the member in place of the radius.
Assuming that the oxygen content of the center portion A of the above silicon nitride sintered body is C1 mass % and the oxygen content of the outer peripheral portion B is C2 mass %, the oxygen content C2 of the outer peripheral portion B falls within a range of (C1-2) to (C1-0.2). Therefore, the oxygen contents satisfy a relation of (C1-2)xe2x89xa6C2xe2x89xa6(C1-0.2) less than C1.
The difference in the oxygen contents (difference in content of 0.2 mass % or more) between the center portion A and the outer peripheral portion B can be achieved by a manufacturing method of the present invention as described in detail later on. That is, above the different content distribution is achieved through a method comprising the steps of: heating a molded compact of silicon nitride powder to a temperature to some extent prior to attaining a temperature at which a final densification-sintering of the silicon nitride was started; and performing a step of holding the compact at such temperature for a predetermined period of time, so that gas components such as oxygen and SiO2 contained in the sintered body are migrated towards outside and discharged from the sintered body.
The formation of the low-oxygen-content region to the outer peripheral portion B of the silicon nitride sintered body means that a residual oxygen content in the outer peripheral portion B and an amount of SiO2 to be generated by combining oxygen with Si are small. Accordingly, an inversion depth (penetration depth) of the defects such as pores and/or cracks resulting from the gas components such as oxygen and SiO2 or the like can be decreased to be shallow. Therefore, the generation of defective product resulting from these defects can be effectively suppressed, and the cost and manpower required for the surface working the sintered body can be also reduced. In view of these facts, the manufacturing cost of the silicon nitride sintered body and the wear resistant member using the sintered body can be remarkably reduced.
In this connection, when the difference of the oxygen content C1 of the center portion A and the oxygen content C2 of the outer peripheral portion B is less than 0.2 mass %, effects as the low-oxygen-content region can not be imparted to the outer peripheral portion B. On the other hand, the oxygen content difference exceeding 2 mass % means that the oxygen content of the outer peripheral portion B is extremely reduced, so that there may be a fear that a densification of the outer peripheral portion B cannot be sufficiently advanced, whereby the strength and the wear resistance of the outer peripheral portion B are adversely lowered. In this regard, it is preferable to set the difference in the oxygen contents between the center portion A and the outer peripheral portion B to within a range 0.5 to 1.5 mass %.
Further, in the silicon nitride sintered body preferably used in the present invention, an oxygen content C3 of an intermediate portion (region C in the silicon nitride sintered body shown in FIG. 1) between the center portion A and the outer peripheral portion B has a content difference of at most 1 mass % with respect to the oxygen content C1 of the center portion A. As described above, the silicon nitride sintered body used in the present invention has a content gradient (concentration gradient) in which the oxygen content is gradually reduced from the center portion A towards the outer peripheral portion B. Due to this structure, a percent defective of the silicon nitride sintered body and the cost or manpower required for the surface working the sintered body can be further suppressed.
Furthermore, in the silicon nitride sintered body used in the present invention, it is preferable that a concrete oxygen content (total oxygen content) is 6 mass % or less as an average value in an entire sintered body. When the total oxygen content exceeds 6 mass %, there is a large fear of deteriorating the inherent characteristics of the silicon nitride sintered body. In addition, it is also preferable that a concrete oxygen content of the outer peripheral portion B is within a range of 3 to 4 mass % in view of suppressing the generation of defects such as pores and cracks resulting from the gas components such as oxygen and SiO2 or the like. Further, in order to also decrease the invasion depth of the defects, the above range of the total oxygen content is effective. It is particularly preferable that the above oxygen content is at most 4.5 mass %.
Note, each of the above oxygen contents of the center portion A, the intermediate portion C and the outer peripheral portion B is measured in such a manner that three measuring points are arbitrarily selected from the respective regions, then an oxygen content at the respective measuring points is measured by means of EPMA (electron probe micro analyzer), and the measured values are averaged to represent the oxygen content of the respective portions. The total oxygen content in the sintered body is a value measured by means of an oxygen analyzer based on an inert gas fusion-infrared-ray absorption method.
By the way, the term xe2x80x9ctotal oxygen contentxe2x80x9d specified in the present invention denotes a total amount in terms of wt % (mass %) of oxygen constituting the silicon nitride sintered body. Accordingly, when the oxygen exists in the silicon nitride sintered body as compounds such as metal oxide, oxidized nitride or the like, the total oxygen content is not an amount of the metal oxide (and oxidized nitride) but an amount of oxygen in the metal oxide (and the oxidized nitride).
In a preferable embodiment of the present invention, as described above, the silicon nitride sintered body is formed to provide a difference in the oxygen contents between the center portion A and the outer peripheral portion B, and to have a content gradient in entire sintered body. However, with respect to metal components other than oxygen in the compounds added as the sintering assistant agent, such metal components are uniformly distributed in the sintered body as in the same manner as the conventional sintered body, so that the content difference in the metal components between the center portion A and the outer peripheral portion B is set to 0.2 mass % or less.
As described above, in the silicon nitride sintered body used in the present invention, unnecessary oxygen is removed to the utmost, while the distribution of the metal components other than oxygen is set to the same as in the conventional sintered body. Therefore, a basic micro-structure of the sintered body comprising silicon nitride crystal grains and grain boundary phases (glass phases) existing between the crystal grains is maintained as it is, so that the inherent characteristics such as strength, hardness, fracture toughness value, sliding property (rolling life characteristics) or the like of the silicon nitride sintered body are maintained as they are.
Regarding hardness required for the wear resistant member, it is preferable that the wear resistant member exhibits a hardness of 1200 or more in terms of Vickers"" hardness (Hv). When the hardness of the silicon nitride sintered body is less than 1200 Hv, the wear resistance is remarkably lowered. In particular, it becomes impossible to sufficiently satisfy the sliding property (rolling life characteristic) required for the bearing balls. It is further preferable that the hardness of the silicon nitride sintered body is 1300 Hv or more.
A method of manufacturing the wear resistant member composed of silicon nitride sintered body of the present invention is characterized by comprising the steps of: preparing a material mixture by adding 1 to 10 mass % of a rare earth element in terms of the amount of an oxide thereof to a silicon nitride powder containing at most 1.5 mass % of oxygen and 75-97 mass % of xcex1-phase type silicon nitride and having an average grain size of 1.0 xcexcm or less; molding the material mixture to form a compact; degreasing the compact; heating and holding the compact at a temperature of 1,250-1,600xc2x0 C. for a predetermined period of time on the way to a sintering step; and conducting a main sintering for the compact at a temperature of 1,650-1,850xc2x0 C. thereby to form a wear resistant member composed of silicon nitride sintered body.
In the above method of manufacturing the wear resistant member, when the sintered body is moderately cooled at a cooling rate of at most 100xc2x0 C. per hour until the sintering temperature is reduced to a point at which a liquid phase formed by the rare earth element during the sintering step solidifies, a size of the pore can be further reduced.
Further, in the above manufacturing method, it is preferable that at most 5 mass % of at least one of aluminum and magnesium in terms of the amount of oxide thereof is added to the silicon nitride powder. Furthermore, it is also preferable that at most 5 mass % of aluminum nitride is added to the silicon nitride powder. Still further, it is also preferable that at most 5 mass % of at least one element selected from the group consisting of Ti, Hf, Zr, W, Mo, Ta, Nb and Cr in terms of oxide is added to the silicon nitride powder.
Furthermore, it is also preferable that the method further comprises a step of conducting a hot isostatic pressing treatment (HIP) to the silicon nitride sintered body in non-oxidizing atmosphere of 300 atm or more at a temperature of 1,600-1,850xc2x0 C. after completion of the sintering step.
According to the above manufacturing method, the silicon nitride sintered body constituting the wear resistant member is prepared in such a manner that the silicon nitride molded body is subjected to a holding operation under a predetermined conditions, followed by being subjected to a main sintering operation, so that the oxygen content of the sintered body is effectively decreased and the generation of pores resulting from the oxygen is suppressed whereby it becomes possible to finely minimize the maximum pore size.
Further, the pores that are liable to be starting points of fatigue breakage at the time of a stress being applied thereto are reduced, so that the wear resistant member excellent in fatigue life and durability can be obtained.
Furthermore, even if a deoxidizing function is advanced due to the holding operation, the sintering property is improved thereby to reduce the pores, so that there can be provided a silicon nitride wear resistant member in which the total oxygen content is at most 6 mass %, preferably at most 4.5 mass %, grain boundaries containing rare earth element or the like are formed in the silicon nitride crystal structure, the maximum size of the pores existing in the grain boundary phase is 0.3 xcexcm or less, porosity is 0.5 vol. %, three point bending strength at room temperature is 900 MPa or more, fracture toughness of 6.5 MPaxc2x7m1/2 or more and crush strength is 200 MPa or more, and having an excellent mechanical properties.
To achieve good sintering characteristic, high bending strength, high fracture toughness value and long rolling life of the product, the silicon nitride fine powder which is used in the method of the invention and contained as a main component in the sintered body constituting the ceramic substrate of the invention preferably contains at most 1.5 mass %, preferably, 0.5-1.2 mass % of oxygen and 75-97 mass %, more preferably, 80-95 mass % of alpha-phase type silicon nitride, and, further the powder has fine grains, that is, an average grain size of at most 1.0 xcexcm, more preferably about 0.4-0.8 xcexcm.
When the silicon nitride powder having an impurity oxygen content exceeding 1.5 mass % is used, although the oxygen content difference between the center portion and the outer peripheral portion is increased, the oxygen content of entire sintered body and porosity are also increased, so that the strength of the silicon nitride sintered body is liable to lower. In other words, even if the silicon nitride powder contains an impurity oxygen at an amount up to 1.5 mass %, the present invention can provide sufficient effects. Therefore, it is not always necessary to use silicon nitride powder having a high purity. Also in view of this advantage, a cost-down can be realized. The more preferable oxygen content of the silicon nitride material powder is within a range of 0.5 to 1.2 mass %.
In this connection, as the silicon nitride material powder, two types of xcex1-phase type Si3N4 powder and xcex2-phase type Si3N4 powder have been known. However, when a sintered body is formed from the xcex1-phase type Si3N4 powder, there is a tendency that a strength is liable to be insufficient. In contrast, in case of the xcex2-phase type Si3N4 powder, although a high temperature is required for the sintering operation, there can be obtained a sintered body having a high strength and a structure in which a number of silicon nitride fibers each having a large aspect ratio are tangled in a complicate manner. Therefore, in the present invention, it is preferable to sinter xcex1-phase type Si3N4 material powder at a high temperature, and to convert into a sintered body mainly composed of xcex2-phase type Si3N4 crystal grains.
A reason why a blending amount of xcex1-phase type Si3N4 powder is limited to a range of 75-97 mass % (wt %) is as follows. That is, when the amount is set to a range of 75 mass % or more, a bending strength, fracture toughness and rolling life of the Si3N4 sintered body are greatly increased thereby to further improve the excellent characteristics of the silicon nitride. On the other hand, the amount is limited to at most 97 mass % in view of the sintering property. It is more preferable to set the range to 80-95 mass %. The range to 85-90 mass % is further more preferable.
As a result, in order to achieve a good sintering characteristic, high bending strength, high fracture toughness and long rolling life of the product, as a starting material powder of the silicon nitride, it is preferable to use the silicon nitride fine powder containing at most 1.5 mass %, preferably, 0.5-1.2 mass % of oxygen, and at least 90 mass % of alpha-phase type silicon nitride, and further the powder has fine grains, that is, an average grain size of at most 1.0 xcexcm, more preferably about 0.4-0.8 xcexcm.
In particular, the use of a fine powder of silicon nitride having an average grain size of 0.7 xcexcm or less facilitates forming a dense sintered body having a porosity of at most 0.5% by volume without requiring a large amount of a sintering assistant agent. The porosity of the sintered body can be easily measured in accordance with a Archimedes"" method.
A total oxygen content contained in the silicon nitride sintered body constituting the wear resistant member of the present invention is specified to 6 mass % or less. When the total oxygen content in the sintered body exceeds 6 mass %, a maximum size of the pore formed in the grain boundary phase is disadvantageously increased, and the pore is liable to be a starting point of a fatigue failure, thereby to lower the rolling (fatigue) life of the wear resistant member. A preferable range the total oxygen content is 4.5 mass % or less.
By the way, the term xe2x80x9ctotal oxygen content of the sintered bodyxe2x80x9d specified in the present invention denotes a total amount in terms of mass % of oxygen constituting the silicon nitride sintered body. Accordingly, when the oxygen exists in the silicon nitride sintered body as compounds such as metal oxide, oxidized nitride or the like, the total oxygen content is not an amount of the metal oxide (and oxidized nitride) but an amount of oxygen in the metal oxide (and the oxidized nitride).
The maximum pore size formed in the grain boundaries of the silicon nitride sintered body constituting the wear resistant member of the present invention is specified to 0.3 xcexcm or less. When the maximum pore size exceeds 0.3 xcexcm, the pore is liable to particularly be a starting point of a fatigue failure, thereby to lower the rolling (fatigue) life of the wear resistant member. A preferable range of the maximum pore size is 0.2 xcexcm or less.
Examples of the rare earth element to be added as a sintering assistant agent to a silicon nitride powder are Y, Ho, Er, Yb, La, Sc, Pr, Ce, Nd, Dy, Sm and Gd. Such a rare earth element may be added to the silicon nitride powder in the form of an oxide thereof or a substance which is changed into an oxide thereof during the sintering process. Two or more kinds of such oxide or substance may be added to the silicon nitride powder. Such a sintering assistant agent reacts with the silicon nitride powder so as to form a liquid phase and thereby serves as a sintering promoter.
The amount of a sintering assistant agent to be added to the material powder is set to be within a range of from 1 to 10 mass % in terms of the amount of an oxide thereof. If the amount is less than 1 mass %, the sintered body fails to achieve a sufficiently high density and high strength. In particular, when an element which has a large atomic weight like lanthanoid is used as the rare earth element at above less amount, a sintered body having a relatively low strength and relatively low thermal conductivity is formed.
On the other hand, if the amount is more than 10 mass %, an excessively large portion of the grain boundary phase is formed, and the generation of pore is increased, thereby reducing the strength of the sintered body. For this reason, the amount of a sintering assistant agent is within the range described above. For the same reason described above, the more preferred range of the amount of a sintering assistant agent is 2 to 8 mass %.
In the present invention, at least one of oxides (Al2O3, MgO) of aluminum (Al) and magnesium (Mg) to be used as an optional addition component promotes a function of the above rare earth element as sintering promoter thereby to enable the sintered body to be densified at a low temperature range. In addition, aluminum oxide (Al2O3) and magnesium oxide (MgO) have a function of controlling a grain growth in the crystal structure of the sintered body thereby to increase the mechanical strengths such as bending strength and fracture toughness value of Si3N4 sintered body.
Therefore, at least one of the oxides (Al2O3, MgO) is added to the material powder at an amount of at most 5 mass %. If the addition amount of at least one of Al and Mg is less than 0.2 mass % in terms of an oxide thereof, the sintered body fails to achieve a sufficiently addition effect. On the other hand, if the amount is excessively large so as to exceed 5 mass %, the oxygen content of the sintered body is disadvantageously increased. For this reason, the addition amount is set to 5 mass % or less, and a preferable range of the amount of the oxide is set to 0.2-5 mass %, more preferably to 0.5-3 mass %.
Further, aluminum nitride (AlN) may also be added as an optional component at a predetermined amount. AlN has a function of suppressing an evaporation of the silicon nitride during the sintering process and further promoting the function of the above rare earth element as sintering assistant agent. Therefore, it is preferable to add AlN at amount of 5 mass % or less.
When the addition amount of AlN is less than 0.1 mass %, a sintering operation at higher temperature is required. On the other hand, when the addition amount is excessively large to exceed 5 mass %, an excessively large amount of grain boundary phase is formed or AlN begins to solid-dissolve into the silicon nitride thereby to disadvantageously increase the pores, and the porosity of the Si3N4 sintered body is also increased. For this reason, the addition amount of AlN is set to the range of 5 mass % or less. In particular, in order to secure good performances together with sintering property, strength and rolling life, it is preferable to set the addition amount of AlN to a range of 0.1-3 mass %.
Further, in the present invention, Ti, Hf, Zr, W, Mo, Nb and Cr may also be added as another optional components at a predetermined amount. These elements to be used as another addition component are added to the substrate as oxides, carbides, nitrides, suicides and borides thereof. These compounds promote the sintering assistant effect of the rare earth element, and promote dispersion thereof in the crystal structure so as to enhance the mechanical strength of the silicon nitride (Si3N4) sintered body. Among them, compounds of Ti and Mo are particularly preferred.
If the amount of these compounds contained is less than 0.1 mass %, the substrate fails to achieve a sufficiently addition effect. On the other hand, if the amount is excessively large so as to exceed 5 mass %, the mechanical strength and rolling life of the sintered body are disadvantageously lowered. For this reason, the preferred range of the amount of these compounds contained is at most 5 mass %. In particular, the amount is more preferably set to a range of 0.2-3 mass %.
The above compounds such as Ti and Mo or the like also serve as light blocking agents (light shielding agents). More specifically, they color the silicon nitride type ceramic sintered body black and thus provides it with an opacity.
Further, since the porosity of the sintered body has a great influence on the rolling life and strength of the wear resistant member, so that the sintered body is manufactured so as to provide the porosity of 0.5 vol. % or less. If the porosity exceeds 0.5% by volume, the pore to be a starting point of the fatigue failure is rapidly increased, thereby to lower the strength of the sintered body and shorten the rolling life of the wear resistant member.
To ensure that the porosity of the silicon nitride sintered body is limited to at most 0.5% by volume and a maximum size of the pore existing in the grain boundary phase formed in the silicon nitride crystal structure is 0.3 xcexcm or less, and having a total oxygen content of 4.5, mass % or less and a predetermined rolling life when the a thrust-type rolling abrasion testing machine is used, it is important that a silicon nitride molded compact prepared from the above material is degreased, followed by holding the compact at a temperature of 1250-1600xc2x0 C. for 0.5-10 hours on the way to a sintering step, followed by sintering the compact by normal-pressure-sintering method or pressured-sintering method at temperature of 1,650-1,850xc2x0 C. for about 2-10 hours.
Furthermore, when the sintered body immediately after the sintering step is moderately cooled at a cooling rate of 100xc2x0 C. per hour or slower, the size of the pore can be further minimized.
In particular, by holding the molded compact at a temperature of 1250-1600xc2x0 C. for 0.5-10 hours on the way to a sintering step, an oxygen concentration in the generated liquid phase (grain boundary phase) is reduced thereby to rise a melting point of the liquid phase, so that the generation of bubble-shaped pore, which is caused when the liquid phase melts, can be suppressed. Simultaneously, the maximum size of the pore is minimized to be fine, so that it becomes possible to improve the rolling life of the sintered body.
Although the above holding operation of the compact on the way to the sintering step exhibits a remarkable effect when the compact is subjected to the treatment in vacuum atmosphere at temperature of 1350-1450xc2x0 C., almost the same effect can be also obtained when the compact is subjected to the treatment in nitrogen atmosphere at temperature of 1500-1600xc2x0 C.
In addition, when the sintered body is moderately cooled at the cooling rate of 100xc2x0 C. per hour or slower in a temperature range from the sintering temperature to a temperature at which the liquid phase solidifies, the reduction of the oxygen concentration in the liquid phase is further promoted, so that there can be obtained a sintered body in which the rolling life is improved.
If the sintering temperature is lower than 1,650xc2x0 C., the sintered body fails to achieve a sufficiently high density; more specifically, the porosity becomes greater than 0.5 vol %, thereby reducing both the mechanical strength and the rolling life of the sintered body to undesired levels. On the other hand, if the sintering temperature exceeds 1,850xc2x0 C., the silicon nitride component per se becomes likely to evaporate or decompose. In particular, if pressured-sintering process is not performed but the sintering process is performed under the normal pressure, the decomposition and evaporation of the silicon nitride component may occur at about 1,800xc2x0 C.
The rate of cooling a sintered body immediately upon completion of the sintering operation is an important control factor to achieve a reduction of the pore size and crystallization of the grain boundary phase. If the sintered body is rapidly cooled at a cooling rate higher than 100xc2x0 C. per hour, the grain boundary phase of the sintered body structure becomes an amorphous phase (glass phase) and, therefore, the reduction of the oxygen content in the liquid phase formed in the sintered body becomes insufficient. Thereby, the rolling life characteristic of the sintered body is disadvantageously lowered.
The sufficiently broad temperature range in which the cooling rate must be precisely controlled is from a predetermined sintering temperature (1,650-1,850xc2x0 C.) to the solidifying point of the liquid phase formed by the reaction of the sintering assistant agent as described above. The liquid phase solidifies at about 1,600-1,500xc2x0 C. if the sintering assistant agent as described above is used.
By maintaining the cooling rate at 100xc2x0 C. per hour or slower, preferably, 50xc2x0 C. per hour or slower, more preferably, 25xc2x0 C. per hour or slower, at least in a temperature range from the above sintering temperature to the solidifying point of the liquid phase, the total oxygen content of the sintered body is 6 mass % or less, the maximum size of the pore is 0.3 xcexcm or less, and porosity becomes 0.5% or less, thus achieving a silicon nitride sintered body excellent in rolling life characteristics and durability. When the above moderate cooling operation is combined with the holding operation as described above, further effective results can be obtained.
A silicon nitride sintered body constituting the wear resistant member according to the present invention can be produced by, for example, the following processes. A material mixture is prepared by adding predetermined amount of a sintering assistant agent, a required additive, such as an organic binder, and a compound of Al, Mg, AlN, Ti or the like, to a fine powder of silicon nitride which has a predetermined fine average grain size and contains very small amount of oxygen. The material mixture is then molded into a compact having a predetermined shape. As a method of molding the material mixture, conventional molding methods such as single-axial pressing method, the die-pressing method or the doctor-blade method, rubber-pressing method, CIP (cold isostatic pressing) method or the like can be applied.
In a case where the molded compact is prepared through the above press-molding method, in order to particularly form a grain boundary hardly causing the pores, it is preferable to set the molding pressure for the material mixture to 120 MPa or more. When the molding pressure is less than 120 MPa, there are easily formed portions to which rare earth element to be a component mainly constituting the grain boundary is agglomerated, and the compact cannot be sufficiently densified, so that there is obtained a sintered body with many crack-formations.
Further, the above agglomerated portion of the grain boundary is liable to become a starting point of fatigue failure, thus lowering the life and durability of the wear resistant member. On the other hand, when the molding pressure is set to an excessively large value so as to exceed 200 MPa, a durability of the molding die is disadvantageously lowered, and it cannot be always said that the productivity is good. Therefore, the above molding pressure is preferably set to a range of 120-200 MPa.
After the above molding process, the molding compact is heated and maintained at 600-800xc2x0 C. for 1-2 hours in a non-oxidizing atmosphere or at 400-500xc2x0 C. for 1-2 hours in the air, thereby degreasing the compact, that is, thoroughly removing the organic binder component added in the material mixture preparing process:
Next, on the way to the step of sintering the degreased compact, a pressure in a sintering furnace is reduced thereby to conduct a vacuum treatment.
That is, the degreased compact is heated up to a temperature range of 1250-1600xc2x0 C. in a vacuum of 0.01 Pa or less and then the compact is held the above temperature of 1250-1600xc2x0 C. for 0.5-10 hours. By conducting such the vacuum treatment, the gas components such as oxygen and SiO2 contained in the sintered body can migrated towards outside and discharged from the sintered body. In this connection, since the discharge of the gas components is firstly caused at outer peripheral portion of the sintered body, it becomes possible to finally reduce the oxygen content at the outer peripheral portion.
When the temperature for the vacuum treatment is less than 1250xc2x0 C., the discharge of the gas components cannot be sufficiently advanced, so that there may be a fear that the oxygen content at the outer peripheral portion of the final sintered body cannot be sufficiently reduced. On the other hand, when the temperature for the vacuum treatment exceeds 1600xc2x0 C., the temperature becomes substantially the same as that of main sintering operation. In this case, the densification is started from the outer peripheral portion at an early stage of the sintering operation, so that there may be a case where the removal of the gas components at an intermediate portion or the like cannot be performed.
The same disadvantages are raised with respect to also a holding time (retention time) of the sintered body at the vacuum treatment. Namely, when the holding time is outside the range as specified above, there may be also a fear that the discharge of the gas components is insufficient, or the discharge of the gas components is excessively advanced.
Next, after the above vacuum treatment (holding operation), the compact is then sintered by normal-pressure-sintering method or pressured-sintering method at a temperature of 1,650-1,850xc2x0 C. in a sintering atmosphere of inert gas such as argon gas or nitrogen gas. As the pressured-sintering method, various press-sintering methods such as a pressurized-atmosphere sintering method, hot-pressing method, HIP (hot isostatic pressing) method or the like can be utilized.
Note, the above sintering step may be successively performed after the vacuum treatment step, or may be also performed in such a manner that the temperature of the sintering furnace is once lowered to a room temperature or a temperature close to the room temperature, then nitrogen gas or the like is introduced into the furnace, thereafter heating the compact up to the sintering temperature.
In addition, when the silicon nitride sintered body is further subjected to a hot isostatic pressing (HIP) treatment under a temperature condition of 1,600-1,850xc2x0 C. in non-oxidizing atmosphere of 300 atm or higher, the influence of the pore constituting a starting point of fatigue failure of the sintered body can be further reduced, so that there can be obtained a wear resistant member having a further improved sliding property and rolling life characteristics.
In particular, when the above silicon nitride sintered body is applied to bearing members such as bearing balls, it is effective to perform HIP (hot isostatic pressing) treatment after the normal-pressure-sintering treatment or the pressured-sintering treatment.
The silicon nitride wear resistant member produced by the above method achieves a total oxygen content of 6 mass % or less, a porosity of 0.5% or less, a maximum pore size of 0.3 xcexcm or less, and excellent mechanical characteristics, that is, a three-point bending strength (at room temperature) of 900 MPa or greater.
Further, there can be also obtained a silicon nitride wear resistant member having a crush strength of 200 MPa or more and a fracture toughness of 6.5 MPaxc2x7m1/2 or more.
Further, according to the manufacturing method of the present invention, there can be obtained the silicon nitride sintered body (the wear resistant member of this invention) having a hardness of 1200 Hv or more and the outer peripheral portion provided with an oxygen content difference i.e. the outer peripheral portion of which oxygen content is 0.2-2 mass % lower than that of the center portion of the sintered body, with a good reproducibility.
According to the silicon nitride wear resistant member and the method of manufacturing the member according to the present invention, the silicon nitride sintered body is manufactured in such a manner that the molded compact is subjected to a predetermined holding treatment on the way to the sintering step, thereafter subjected to a main sintering operation, so that the oxygen content in the sintered body is reduced and the generation of the pore is effectively suppressed thereby to enable the maximum pore size to be extremely small. Therefore, there can be obtained a wear resistant member having an excellent rolling life property and a good durability.
Therefore, when a bearing device is prepared by using this wear resistant member as rolling bearing material, good sliding/rolling characteristics can be maintained for a long time of period, and there can be provided a rotation machine having excellent operational reliability and durability. Further, as an example of another application, the wear resistant member of this invention can be applied to various fields such as engine parts, various tool material, various rails, various rollers or the like.
That is, although the silicon nitride sintered body used in the present invention can be used for various applications, the sintered body is particularly effective to a material for constituting the wear resistant member. An example of the wear resistant member to which this silicon nitride sintered body is applicable may include bearing member, various rollers for rolling machine, compressor vane, gas turbine blade, engine parts such as cam roller or the like. Among these examples, the present invention is particularly effective with respect to the bearing members such as bearing ball of which entire surface constitutes a sliding portion. In more particular, the present invention is significantly effective to a bearing ball having a relatively large diameter of 9 mm or more.
By the way, it goes without saying that, if necessary, the silicon nitride sintered body to be used as the wear resistant member may be subjected to finishing works such as surface grinding or coating treatment or the like. In other words, when the silicon nitride sintered body can be used as the wear resistant member as it is, the silicon nitride sintered body directly constitutes the wear resistant member.